Combine water and milk in a medium saucepan; bring to a boil. Add margarine and garlic; stir until margarine melts. Stir in grits and salt; cover, reduce heat, and simmer 15 minutes or until creamy, stirring occasionally. Stir in cheese and whipping cream. Cook, uncovered, over medium heat, stirring frequently, 12 to 15 minutes or until grits are thickened.